N compounds are essential to all forms of life. Most important biomolecules contain
N in a form similar to ammonia (-3 oxidation state). Almost all such N is called ammoniacal
N - one of the hydrogen atoms combined with the N atom of ammonia being replaced by a
carbon atom, e.g., C-NH2. N is a vital component of proteins, peptides, enzymes,
energy-transfer molecules (ATP, ADP), and genetic material (RNA and DNA) - materials that are vital to all
organisms.

Whereas the amount of N needed by animals, microorganisms, and plants varies
considerably (Table 3), the amounts of N required are always great enough to make N fall
into the category of being an essential macronutrient (needed in large amounts relative to other
important essential nutrients such as: calcium (Ca), phosphorus (P), potassium (K), sulfur (S), and magnesium, (Mg)). In all
cases, the nutritional requirements for N are exceeded only by those of carbon (C), hydrogen
(H), and oxygen (O).

Certain bacteria also use N compounds in respiration (energy production). In all organisms,
respiration is an oxidation-reduction (redox) reaction involving an oxidant (electron acceptor) and a
reductant (electron donor). Aerobes (including humans) use O as the electron acceptor. The energy
available from various reactions that are mediated by organisms is given in the first seven rows of Table 4,
where "CH2O" (the generic formula for carbohydrate) indicates organic matter. (The reactions shown in
Table 4 are the net result of complex multi-step biochemical processes.)

Table 4. Reduction and oxidation reactions used in respiration

Reaction

Name

Free Energy
Change
(kJ/mol)*

1/4O2(g)+ 1/4CH2O ® 1/4CO2(g)+ 1/4H2O

Aerobic
respiration

-119

1/5NO3- + 1/4CH2O + 1/5H+® 1/10N2 + 1/4CO2(g) +
7/20H2O

Denitrification

-113

½ MnO2(s) + 1/4H2O + H+® ½ Mn2+ + 1/4 CO2(g) +
3/4H2O

Manganese
reduction

-97†

1/8NO3- + 1/4H+ + 1/4 CH2O ® 1/8NH4+ + 1/4CO2(g) +
1/8H2O

Nitrate
reduction

-76

Fe(OH)3(s) + 1/4CH2O + 2H+® Fe2+ + 1/4CO2(g) +
11/4H2O

Iron reduction

-47†

1/8SO42- + 1/4CH2O + 1/8H+® 1/8HS- + 1/4CO2(g) +
1/4H2O

Sulfate
reduction

-21

1/4CH2O ® 1/8CO2(g)+ 1/8CH4(g)

Methane
fermentation

-18

1/4CH2O + 1/4H2O ® 1/4CO2(g) + 1/2H2(g)

Hydrogen
fermentation

-1

1/6NH4+ + 1/4O2(g) ® 1/6NO2- + 1/3H+ + 1/6H2O

Nitrification

-45

1/2NO2- + 1/4O2(g) ® 1/2NO3-

Nitrification

-38

Notes:

Source: Morel and Hering 1993

*

25ºC, pH 7

†

1 M dissolved Mn or Fe

The electron donor that yields the most energy usually determines the predominant type of
respiration in a particular environment. Therefore, when oxygen is present, aerobic respiration is the
predominant form of respiration. Denitrification is the second most energetic reaction in Table 4. Therefore,
when oxygen becomes depleted, then NO3- becomes the preferred electron acceptor, followed by
manganese and iron oxides, and finally sulfate (SO42-). This sequence of redox reactions is observed in
environments that are not in contact with the atmosphere, including sediments, flooded soils, and aquifer
systems. The last two rows of Table 4 show N species as electron donors.